1. Field of the Invention
The present invention relates to a process for preparing saturated carboxylic acids having from one to four carbon atoms, in particular acetic acid, by gas-phase oxidation of 2-butanone using a coated catalyst.
2. The Prior Art
It is known that saturated carboxylic acids having from one to four carbon atoms and in particular acetic acid can be prepared by gas-phase oxidation of short-chain aliphatic compounds, particularly butenes and alcoholic or ketonic oxidation products obtainable therefrom, by means of a catalyst. However, no process which is fully satisfactory from economic and production engineering points of view has been found hitherto.
DE-1 643 822 C3 describes a process for preparing acetic acid by catalytic gas-phase oxidation of short-chain aliphatic compounds by means of oxygen using aluminum vanadate and titanium vanadate catalysts. These catalysts are prepared by precipitation of the mixed oxides from the corresponding solutions and are used as granulated all-active catalysts. Disadvantages of these catalysts are the high degree of total oxidation and the difficulty of controlling heat removal. As a result, a yield of only 75% and a maximum acetic acid production of 64 g per liter of catalyst and hour are achieved in the prior art oxidation of methyl ethyl ketone (2-butanone). Furthermore, a crude acid having a concentration of only about 12% by weight is obtained. This requires a very costly concentration and purification in order to obtain a commercial >99% strength pure acetic acid.
DE-1 643 824 C describes a process for preparing concentrated acetic acid by catalytic gas-phase oxidation of butenes and alcoholic or ketonic oxidation products obtainable therefrom. The catalysts used are aluminum vanadate, antimony vanadate, tin vanadate and titanium vanadate catalysts in a sequence of individual reactors connected in series. These catalysts are prepared by precipitation of the mixed oxides from the corresponding solutions and are used as granulated all-active catalysts. Disadvantages of this process and the catalysts described therein are the high degree of total oxidation and the difficulty of controlling heat removal. As a result a yield of only 66% and a maximum acetic acid production of 126 g per liter of catalyst and hour are achieved in the prior art oxidation of methyl ethyl ketone (2-butanone). Furthermore, a crude acid having a maximum concentration of only 34% by weight is obtained. This requires, in addition to the very complex series arrangement of four reactors having their own introduction of starting material and two condensers, very costly concentration and purification of the crude acid in order to obtain a commercial >99% strength pure acetic acid.
DE-1 643 824 C describes the use of 2-butanone (methyl ethyl ketone) as a starting material which gives poorer results with respect to the acetic acid yield and the achievable space-time yield. It also describes the poorer results for the crude acid concentration compared to the use of n-butenes such as 1-butene, trans-2-butene, cis-2-butene.
DE-1279011 B describes a process for preparing acetic acid by catalytic gas-phase oxidation of butene by means of oxygen using aluminum vanadate and titanium vanadate catalysts. These catalysts are likewise prepared by precipitation of the mixed oxides from the corresponding solutions, with the mixed oxides being able, if desired, to be mixed with inert materials such as silica. The catalyst is used as a finely divided powder in fluidized-bed reactors. A disadvantage of such all-active catalysts is the high degree of total oxidation.
To improve the yield obtained using such catalysts, DE-2016681 A proposes that the catalysts be treated with an oxidant prior to calcination. DE-A 2354425 C3 (U.S. Pat. No. 3,954,857) proposes treatment of the calcined titanium-vanadium mixed catalyst with hydrochloric acid to improve the selectivity. The catalysts are used as all-active catalysts, if desired in admixture with inert support materials such as silica.
A further starting point known from the prior art for improving the activity of titanium-vanadium mixed catalysts in the gas-phase oxidation of butenes to acetic acid is the use of titanium dioxide in a defined crystal form or with a defined surface area. DE-A 2026744 (U.S. Pat. No. 3,917,682) describes titanium-vanadium mixed catalysts whose titanium dioxide component is predominantly present as rutile. The catalysts can be used in powder form or as pressed shaped bodies. U.S. Pat. No. 4,448,897 discloses Ti-vanadium catalysts comprising titanium dioxide having a BET surface area of greater than 40 m2/g for butene oxidation. The catalysts are likewise used in powder form or as pressed bodies.
It is also known from the prior art that the selectivity of titanium-vanadium catalysts in the oxidation of butenes can be improved by replacing all or part of the titanium dioxide present by other metal oxides. DE 2110876 A (GB 1333306), for example, describes catalysts comprising oxides of molybdenum, tin and vanadium as active components. The catalysts are used in powder form, and the mixed oxide catalyst can, if desired, also be applied to finely divided support materials such as silicon dioxide. U.S. Pat. No. 4,146,734 discloses the use of vanadium mixed oxides doped with cerium and further transition metal oxides. The catalyst is used as fine granules but can also be applied as a precipitate to finely divided, inert supports.
DE 2235103 C3 discloses titanium-vanadium mixed oxide catalysts for the gas-phase oxidation of butenes in the form of supported catalysts obtained by impregnating a previous shaped porous support with the mixed solution of the catalyst components.
All these catalysts are all-active catalysts in which the active components themselves are used as powder or pressed bodies, or are diluted with finely divided support materials before use as powder or pressed bodies. For the purposes of the present invention, all-active catalysts also include porous supports impregnated right through with an active component as described in DE 2235103 C3, since here, too, the total catalyst volume is catalytically active.
Disadvantages of all these processes and catalysts are the high degree of total oxidation, the difficulty of controlling the oxidation reaction at high space-time yields and the low acetic acid concentration in the crude acid. These disadvantages lead to high production costs for the acetic acid.
EP 0 951 351 and EP 0 960 874 describe a coated catalyst for preparing acetic acid by gas-phase oxidation of hydrocarbons having four carbon atoms (C4-hydrocarbons).
EP 0 960 875 and EP 1 035 101 describe processes for preparing saturated carboxylic acids having from 1 to 4 carbon atoms from saturated and unsaturated C4-hydrocarbons using coated catalysts.
These processes have the disadvantage that although high acetic acid yields are achieved, the volume-based acetic acid productivity (space-time yield) and the concentration of the crude acid are not fully satisfactory. Consequently, implementation of these processes is still associated with high reactor and work-up costs.
It has also been found that increasing the proportion of n-butenes in the raw material or increasing the reactor throughput enables the volume-specific acetic acid productivity of the known processes for producing acetic acid from C4-hydrocarbon mixtures to be increased only insignificantly and/or with reductions in selectivity.